1. Field of the Invention
The invention relates to the field of laser surgery, and in particular, to the thermal treatment of biological tissues with laser pulses.
2. Description of the Prior Art
The illustrated embodiment of the invention is described below in the context of treatment of port wine stain birthmarks in human skin, although the scope of the invention is much broader in that it applies to all types of thermal surgeries. A port wine stain is congenital, progressive, vascular malformation of the dermis involving capillaries and possibly perivenular nerves. Port wine stains occur in approximately three percent of one thousand live births. Although port wine stains may be found anywhere on the body, they mostly appear on the face and are noted over the dermatome distribution of the first and second trigeminal nerves.
In early childhood, port wine stains are faint pink macules, but the lesions tend to darken progressively to red-purple and by middle age, often become raised as a result of the development of vascular papules or nodules and occasionally tumors. The hypertrophy of underlying bone and soft tissue occurs in approximately two-thirds of the patients with port wine stain, and serves to further disfigure the facial features of many children.
The prior art treatments for port wine stain includes scalpel surgery, ionizing radiation, skin grafting, dermabrasion, cryosurgery, tattooing and electrotherapy. Clinical results have been considered unsatisfactory due to the cosmetically unacceptable scarring post treatment. All of these prior art techniques are no longer considered viable treatment options for this reason.
A flashlamp-pumped pulsed dye laser offers a superior approach and therapy due to its ability to selectively destroy cutaneous blood vessels. Light passing through the epidermis is preferentially absorbed by hemoglobin which is the major chromophore in blood in the ectatic capillaries in the upper dermis. The radiant energy is converted to heat causing thermal damage and thrombosis in the targeted vessels. Prior art studies have shown that the flashlamp-pumped pulsed dye laser produce good results in the vast majority of pediatric and adult patients.
Histopathological studies of port wine stains show a normal epidermis overlying an abnormal plexus of dilated blood vessels located on a layer in the upper dermis as diagrammatically depicted in cross sectional view in FIG. 1. The predominate endogenous cutaneous chromophores, absorbing light at the 585 nanometer wavelength produced by flashlamp-pumped pulsed dye laser, are melanin and hemoglobin. Therefore, the overlying epidermal pigment layer comprises a barrier or an optical shield through which the light must first pass to reach the underlying port wine stain blood vessels. The absorption of laser energy by melanin causes localized heating in the epidermis and reduces the light dosage reaching the blood vessels, thereby decreasing the amount of heat produced in the targeted port wine stains and leading to suboptimal blanching of the lesion.
The ratio of heat generated in port wine stains to that of the epidermis is a measure of the relative heating of the port wine stain relative to the epidermis. The best clinical results realized in a port wine stain patient undergoing laser therapy are obtained when the patient's ratio of heat generated in the port wine stain to that in the epidermis is greater than or equal to one. Unfortunately, for many lesions, the threshold for epidermal damage following laser therapy is very close to the threshold for permanent blanching of the port wine stain.
One prior art method which has been tried is the application of ice cubes to the skin surface prior to laser treatment, B. A. Gilchrest et al., "Chilling Port Wine Stains Improves the Response to Argon Laser Therapy," Plast. Reconstr. Surg. 1982; 69:278-83. However, these treatments have not proven entirely satisfactory, nor more importantly led to an improved therapeutic response, that is improved blanching of the port wine stain.
Other prior art attempts to provide surface cooling of the epidermis using plastic bags filled with ice placed on the skin surface for five minutes, compressed freon gas used during irradiation, or chilled water spread directly on the area being irradiated have also been explored, A. J. Welch et al., "Evaluation of Cooling Techniques for the Protection of the Epidermis During ND-YAG Laser Irradiation of the Skin," Neodymium-YAG Laser in Medicine, Stephen N. Joffe editor 1983. However, these studies were done with pig cadaver tissue and normally utilized cooling periods of 2 to 14 seconds. The reported results with freon were good in only 28.5 percent of the cases, in some cases, the skin surface was momentarily frozen, and in others, the freon jet was found to overcool the skin surface.
Therefore, what is needed is some type of methodology or apparatus which can be effectively used to uniformly provide positive results, namely allowing treatment of deeper or selected layers of tissue without nonspecific damage to the upper or nonselected layers.